The invention relates to a static bipolar memory cell having a first and a second transistor, in which the base zone of the first transistor is connected to the collector zone of the second transistor and the base zone of the second transistor is connected to the collector zone of the first transistor, and in which the collector zones are connected to a supply line via a load element.
Such memory cells and memories having such cells are generally known.
In a simple form, the load elements in known cells are formed by resistors which exhibit a current-voltage characteristic which, in principle, is linear.
However, these cells have the disadvantage of a small read/stand-by current ratio (substantially=1). Preferably, however, this ratio is as large as possible, as will be explained hereinafter.
As is known, the cells have two stable states in which a current flows either through the first or through the second transistor depending on the stored information. During operation, the situation will regularly occur that the information stored in a cell need not be read. The current which flows through the cell in this situation, and which is termed stand-by current, mainly determines the stand-by power dissipation, and for this reason is kept as small as possible. The lower limit of the stand-by current is determined by the impedance of the load elements. Below said lower limit stability criteria are no longer satisfied so that the stored information may be lost. During reading, a large current is preferably passed through the cell so as to rapidly charge parasitic capacitances and hence reduce the read time. The maximum value for the read current is also determined by the value of the impedance of the load element.
In order to improve the ratio of read current/stand-by current, it has previously been suggested to use a nonlinear resistive element instead of a linear resistive element. Such a nonlinear resistive element may be formed, for example, by a resistor with a diode connected in parallel, so that a small stand-by current can be passed via the comparatively high resistance and the large read current via the diode. These diodes provide nonlinear loads with which in practice read/stand-by current ratios of 10-20 have been obtained. In a process known per se with an epitaxial layer of one conductivity type on a substrate of the same conductivity type and intermediate buried collector layers of the opposite conductivity type, the diodes can even be obtained automatically, as will become apparent from the description of the Figures hereinafter. The devices obtained in this simple manner show favorable properties in the sense that, as a result of the low stand-by current, dissipation is low, while read time may be small due to the large read current.